Research On Micro-Grid Droop Control Strategy And Harmonic Suppression Methods Based On Virtual Impedance

With the development of social economy and the growth in electricity demand, micro-grid can take full advantage of the new energy, so the development of micro-grid has drawn more and more attention in recent years. Due to the presence of power electronic converters and non-linear loads in micro-grid, effects of harmonics on micro-grid are increasingly significant. And harmonic problems will seriously affect the power quality of the grid. Therefore, in order to establish micro-grid droop control strategy and a harmonic suppression method based on virtual impedance, a droop control based on virtual impedance with a SOGI model is researched in this dissertation.First of all, the power transmission characteristics of the micro-source are analyzed, then the voltage and current loop controller, power calculation unit and droop controller model of micro-source are built. Also the transfer function of micro-source voltage and current loop control model is analyzed in detail using frequency domain method, and the parameters of voltage and current loop controller are designed using pole assignment.Secondly, a virtual impedance loop is added to the conventional droop control strategy, which composed the droop control strategy based on virtual impedance with a SOGI model. Then the influence on system output characteristics after the introduction of virtual impedance loop is depicted using frequency domain method. And according to the closed-loop transfer function of the inverter control system, the stability of droop control model with virtual impedance loop is analyzed. Besides a simulation of micro-grid is presented, which can verify the effectiveness of the droop control method based on virtual impedance.Finally, a droop control based on virtual impedance is analyzed, which can effectively suppress harmonic.And harmonic suppression simulation examples of micro-grid operating in islanded and grid-connected modes with virtual impedance loop are presented in MATLAB/Simulink, in order to verify the the effectiveness of droop control strategy based on virtual impedance in terms of harmonic suppression. In addition, impact of virtual impedance value on harmonic suppression in micro-grid is analyzed.In this dissertation, the droop control based on virtual impedance with a SOGI model is established, which can enable the system stability as well as realize automatic distribution of power when the load changed. Also the harmonic suppression method of micro-grid based on virtual impedance is established, which can effectively suppress micro-grid harmonics and improve the power quality. And the research has certain significance on harmonic suppression of micro-grid.